The present invention relates to industrial filters and, more particularly, to the control of the pressure differential in a vacuum type filter.
Such vacuum type filters normally have a movable filter media. One typical type filter system is shown in U.S. Pat. No. 4,396,505.
The filter apparatus normally comprises a supply of liquid to be filtered which has a discharge into the actual filter tank. Such filter tank is normally of an elongated box configuration and is divided into two sections by a movable filter grid. The liquid to be filtered is inlet into the filter tank and is passed through a filter media supported on the filter grid. The filtered liquid enters the filtrate compartment of the filter tank and is passed back into the industrial system for reuse via a pump. The suspended matter collected from the liquid being filtered is deposited on the filter media. A conveyor system is intermittently activated to transport the filter media with the suspended matter up a ramp like end wall of the filter tank for disposal. This type system is usually referred to as a vacuum type filter because the pump discharging the filtered liquid causes a pressure differential across the filter media and accordingly between the inlet portion of the filter tank and the filtrate compartment of the filter tank.
As a result of the pressure differential across the filter media, it is difficult for the conveyor system to move the filter media upon the clogging of the filter media by collected suspended matter. The pressure differential across the filter media must be relieved before the filter media can be advanced. A sensing device is utilized to determine when the pressure differential across the filter media is excessive due to the build up of contaminents on the filter media. The sensing device will open a valve discharging from a filtered liquid reservoir directly into the discharge pump and the filtrate compartment of the filter tank. The pressure differential across the filter media is thereby relieved and the filter media may be advanced by the conveyor system.
A problem exists in this known method of alleviating the pressure differential. This problem involves an excess flow from the storage tank or filtered liquid reservoir through the vacuum break valve and into the filtrate compartment. Especially when the preceding normal filter flow rate is reduced from near 100% normal flow to near 0% normal flow, and the sensing device opens the vacuum break valve, the flow from the storage tank into the filtrate compartment can be excessive. Such excessive flow can cause an abnormal upward flow from the filtrate compartment into the inlet compartment thereby displacing or actually tearing the filter media. Such an abrupt relief of the pressure differential across the filter media is a serious problem in this type of vacuum filter.
Accordingly, it is an object of the present invention to provide an improved pressure differential control in industrial vacuum type filters.